Add database index for cancelable ci_pipelines on user_id and id

It improves the performance of the query that searches for cancelable
pipelines for a given user.

changelogs/unreleased/ci-improve-users-cancelable-pipelines-query.yml

+---
+title: Add database index for cancelable ci_pipelines on user and id
+merge_request: 56314
+author:
+type: performance

db/post_migrate/20210311093723_add_partial_index_on_ci_pipelines_by_cancelable_status_and_users.rb

+# frozen_string_literal: true
+
+class AddPartialIndexOnCiPipelinesByCancelableStatusAndUsers < ActiveRecord::Migration[6.0]
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+  INDEX_NAME = 'index_ci_pipelines_on_user_id_and_id_and_cancelable_status'
+  INDEX_FILTER_CONDITION = <<~SQL
+    ((status)::text = ANY (
+      ARRAY[
+        ('running'::character varying)::text,
+        ('waiting_for_resource'::character varying)::text,
+        ('preparing'::character varying)::text,
+        ('pending'::character varying)::text,
+        ('created'::character varying)::text,
+        ('scheduled'::character varying)::text
+      ]
+    ))
+  SQL
+
+  disable_ddl_transaction!
+
+  def up
+    add_concurrent_index :ci_pipelines, [:user_id, :id], where: INDEX_FILTER_CONDITION, name: INDEX_NAME
+  end
+
+  def down
+    remove_concurrent_index_by_name :ci_pipelines, INDEX_NAME
+  end
+end

db/schema_migrations/20210311093723

+6af1c870e685b5a7fbc31b658959018c313713f0f35ad776f97d0a804f1c0f05
\ No newline at end of file

db/structure.sql

@@ -22103,6 +22103,8 @@ CREATE INDEX index_ci_pipelines_on_user_id_and_created_at_and_config_source ON c
 
 CREATE INDEX index_ci_pipelines_on_user_id_and_created_at_and_source ON ci_pipelines USING btree (user_id, created_at, source);
 
+CREATE INDEX index_ci_pipelines_on_user_id_and_id_and_cancelable_status ON ci_pipelines USING btree (user_id, id) WHERE ((status)::text = ANY (ARRAY[('running'::character varying)::text, ('waiting_for_resource'::character varying)::text, ('preparing'::character varying)::text, ('pending'::character varying)::text, ('created'::character varying)::text, ('scheduled'::character varying)::text]));
+
 CREATE UNIQUE INDEX index_ci_project_monthly_usages_on_project_id_and_date ON ci_project_monthly_usages USING btree (project_id, date);
 
 CREATE UNIQUE INDEX index_ci_refs_on_project_id_and_ref_path ON ci_refs USING btree (project_id, ref_path);